A while back on Dreams in the Lich House, there was a couple of posts (here and here) regarding what makes a D&D setting good in terms of playability. Although there was some disagreement over which elements count as core and essential to the game, no one has offered a better treatment yet (regardless of what exactly "better" is supposed to mean in this context).
Assuming there exist a number of criteria which absolutely have to be satisfied, we can find the core elements of a D&D setting by analysing how adventures are generally set up, what elements they include (locations, items, etc.), and how they relate to the player characters' actions.
Reading the relevant sections of the books carefully and recalling how my own (and, thanks to actual play reports, others') games usually go, I came to the following:
Freemen, outlaws, or gold diggers, they represent the peculiar social class whose members are not - or are minimally - bound by the laws of Man or God and are capable of going on adventures. Without a society in which this type of individuals can exist (or rather exist opposed to ordinary folks), traditional D&D adventures can hardly take place.
(Actually, there is one case which allows such gameplay: going on missions given by a powerful patron/lord/whatever as mercenaries/secret agents/specialists; at the same time, it outright kills traditional sandbox gaming.)
The world has to include some sort of mythic underworld or other similar place where the fantastique can be found. The setting does not necessarily have to support a megadungeon (as it turns out many people do not use them, despite it being "the" traditional and thus most archetypical type of D&D adventure), but then at least some other smaller adventure locations need to be justified.
Different types of foes, be they humans, demi-humans, aliens, monsters, or mythological creatures. Most of them are assumed to be enemies on sight, others may prove to be useful allies or sources of information. Also, they need to be of different power levels and of distinct abilities to keep encounters interesting and challenging.
The goal of the game, the ultimate motivation for going on adventures; not only does it increase power (cf. 1 gp = 1 XP), but also allows PCs to buy new stuff, improve their gear, and generally raise in social status. Treasure also includes "magic items", regardless of how in the setting they are represented (ancient high-tech items, for instance).
Wilderness vs. Civilisation
Points of light, where the brave/foolish adventurers can equip themselves, spend their cash, and generally hang out between adventures. Everywhere else, however, chaos/evil/uncertainty rules - these areas provide interesting adventure locations, where monsters can be encountered and treasures can be found.
I believe these elements are general enough not to be restrictive regarding creativity and campaign flavour, yet they cover every part of traditional D&D adventuring. I deliberately left out domain management; its excluding is because of my personal lack of experience: first, my campaigns are generally too short to reach the end game; second, I prefer dungeon-crawling above everything else. I know this might not be enough justification, and I am open to be talked into including it, even though most people play D&D with only the aforedetailed (wait, the daily new word has to be in the dictionary already?) elements, and I am sure they play D&D as intended.
Anyhow, what do you think? Are these the only criteria? If not, what else would you include?